PROJECT SUMMARY/ABSTRACT Rates of adolescent substance use (ASU) remain at problematic levels1 despite considerable investments in evidence-based prevention program implementation,2,3 specifically universal family-based ASU prevention programs. To enhance the effects of family-based prevention programs, it is necessary to have a deeper understanding of differential program effectiveness. Traditional assessments conceal the heterogeneity in program effects making it difficult to improve program effectiveness for those who may benefit less than others.4?8 Rather than using single items (e.g., gender) or cumulative indices (e.g., ?family risk?) as moderators of program effectiveness,8,9 the applicant proposes to apply innovative person-centered methods10?12 to study differential program effects by identifying subgroups based on family-level and individual-level risk and protective factors (RPFs). This approach will yield a holistic typology that accounts for the co-occurrence of different RPFs, a more true-to-life depiction of adolescent context than cumulative indices. Typologies can be used to advance understanding in ASU etiology as well as differential intervention effects. The proposed study will use data from the PROSPER Study (R01 DA013709), a large, community randomized trial with longitudinal follow-up data to examine the following three aims: (1) To identify profiles of co-occurring family-level and individual-level RPFs, (2) To understand how RPF profiles predict ASU over time, and (3) To determine for whom the Strengthening Families Program: For Parents and Youth Ages 10-14 works best in the prevention of ASU over time. By addressing each of the above research aims, the proposed study will contribute to knowledge that will help program developers and prevention scientists better serve the larger population as a whole through evaluation of differential intervention effects. The knowledge gained in this study will allow future research projects to improve existing interventions, and develop new interventions, that target the most promising mechanisms for specific subgroups. To better prepare herself for a career conducting ASU prevention research, the applicant also seeks additional training to: (1) gain expertise on ASU etiology and the role of RPFs in this developmental process, (2) deepen understanding of intervention effects on the family ecology and the associated ASU outcomes, (3) learn and apply advanced methods for evaluating RPFs for ASU, (4) strengthen manuscript and grant writing skills and engage in professional activities, and (5) responsibly conduct research in adolescent substance use prevention. The applicant?s career goal is to conduct independent research with the objective of innovating preventive strategies to better serve all program participants to reach population-level effects on substance use prevalence rates in adolescents, and thus prevent additional negative downstream consequences. This dissertation research will lay the foundation for a career-long research program investigating and implementing ASU prevention strategies.